Surface Acoustic Wave devices, generally designated by the acronym SAW devices, find applications in the field of radio-frequency (RF) communications and, in particular, for filter applications.
A SAW device typically comprises a piezoelectric layer and two electrodes in the form of two interdigitated metallic combs deposited on the surface of the piezoelectric layer.
An electrical signal, such as an electrical voltage change applied to an electrode, is converted into an elastic wave, which is propagated at the surface of the piezoelectric layer. The wave is converted once more into an electrical signal on reaching the other electrode.
The choice of the piezoelectric material takes into account the electromagnetic coupling coefficient, which expresses the rate of electromagnetic conversion by the material, and the temperature stability of the oscillation frequency of the piezoelectric material.
SAW devices are very sensitive to variations in temperature, which induce different degrees of expansion of the piezoelectric layer and of the metallic electrodes due to the different coefficients of thermal expansion of these materials.
More precisely, the Temperature Coefficient of Frequency referred to by the acronym TDF and defined as the variation of a given frequency f as a function of the temperature T, is given by the formula:
      TCF    =                            1          f                ⁢                              ∂            f                                ∂            T                              =              TCV        -        CTE                  where    ⁢          :            TCV    =                  1        V            ⁢                        ∂          V                          ∂          T                    
V is the speed of the surface acoustic waves and
CTE is the coefficient of thermal expansion of the piezoelectric material in the direction of propagation of the surface acoustic waves.
Measures already exist for compensating for the effects of temperature on SAW devices.
In particular, the article by Hashimoto et al. [1] provides a review of the various temperature-compensation techniques for SAW devices.
Amongst these different techniques, the following can essentially be distinguished:                (1) a so-called “overlay” technique consisting of covering the surface of the piezoelectric layer and the electrodes with a dielectric material (typically silicon oxide (SiO2)), which exhibits a coefficient of thermal expansion in the opposite sense to that of the piezoelectric layer,        (2) a so-called “wafer-bonding” technique consisting of bonding the piezoelectric layer to a support substance whose coefficient of thermal expansion is as low as possible so as to neutralize the thermal expansion of the piezoelectric layer.        
The support substrate, which may be made, for example, of silicon, of sapphire, of glass or of spinel (MgAl2O4), thus performs a stiffening function of the piezoelectric layer. Given its thickness, the piezoelectric layer is considered to extend to infinity in a direction away from the electrodes, so that the presence of the support substrate does not interfere with the propagation of the surface acoustic waves. Nevertheless, the bonding of the support substrate appears to create spurious resonances at frequencies greater than the principal frequency of the device (see [1], FIG. 5).
Of the materials envisaged for the support substrate in this second technique, silicon seems to be the most promising as it allows integration methods for electronic components at the substrate scale (so-called “wafer level”) to be implemented.
Nevertheless, a significant difference in thermal expansion coefficients exists between the piezoelectric material and silicon (for a crystal of LiTaO3, which is anisotropic, the CTE values are about 4×10−6/° C. and 14×10−6/° C., while the CTE of silicon is of the order of 2.3×10−6/° C.), which affects the stability of the support substrate/piezoelectric layer stack if the latter is exposed to high temperatures during the subsequent steps in the method for manufacture of the surface acoustic wave device. In light of such steps, thermal stability of the piezoelectric layer/support substrate stack must be ensured up to a temperature of about 250° C.
A similar problem arises for bulk (volume) acoustic wave filters and resonators, known by the acronym BAW.
Bulk acoustic wave filters and resonators typically comprise a thin piezoelectric layer (that is, with a thickness in general of less than 1 μm) and two electrodes arranged on each principal face of the thin layer. An electrical signal, such as an electrical voltage change applied to an electrode, is converted into an elastic wave, which is propagated through the piezoelectric layer. This wave is converted once more into an electrical signal on reaching the electrode located on the opposite face.